In general, in electrophotography, after the uniform charging of a surface of an electrostatic latent image bearing member (photoconductor) using corona discharge, the uniformly charged surface of the electrostatic latent image bearing member is exposed to a laser beam to form an electrostatic latent image on the surface of the electrostatic latent image bearing member. The electrostatic latent image is then developed with toner to form a toner image on the surface of the electrostatic latent image bearing member. The toner image is then transferred to a recording medium to produce a high-quality image. Toners for use in the formation of toner images are generally manufactured by mixing a binder resin, such as a thermoplastic resin, with a colorant, a charge control agent, a release agent, and a magnetic material, and subjecting the mixture to kneading, pulverization, and classification. Such toners have an average particle diameter of 5 to 10 μm. In order to impart flowability to the toner, control the amount of electrostatic charge of the toner, and/or improve removability of a residual toner on a photoconductor after transfer, an inorganic fine powder, such as silica and/or titanium oxide, is added to the toner.
From the perspective of energy conservation and size reduction, there is a demand for toner having excellent low-temperature fixability with minimum heating of a fixing roller. However, toners having excellent low-temperature fixability often contain a binder resin having a low melting point or glass transition point or a release agent having a low melting point. Storage of such toners at high temperature therefore generally causes aggregation of toner particles. Furthermore, toner fused on a heated fixing roller tends to cause high-temperature offset.
Various studies have been performed to solve these problems. One proposed toner has tan δ peak temperatures of 50 to 100° C. and 130 to 180° C. in viscoelasticity measurement at a frequency of 1 Hz and a strain of 0.1.
Although the proposed toner has high storage stability, the toner sometimes cannot be fixed at low temperature or causes offset at high temperature in the image formation with an image-forming apparatus for high-speed printing or an image-forming apparatus having a high fixing pressure (nip pressure). Thus, there is a need for toner that has excellent low-temperature fixability and a reduced likelihood of causing offset at high temperature without adversely affecting the storage stability of the toner.